Expanded perlite is increasingly being added in different proportions to various materials, for example for building products and building materials like plasterboard and gypsum/cardboard boards. Since the expanded perlite in general has a bulk density of only 40-80 g/l and serves primarily to make the end product lighter, it usually is metered not by weight but rather by volume. This dosing or metering has been carried out heretofore with the aid of a measuring container into which the material is filled from a storage silo, for example via a conveyor worm. The measurement container has a measurement probe which interrupts the material feed when the desired filling height is reached. This measurement probe is height adjustable so that any desired volume per charge can be established. The measurement container also has a material outlet at its lower end through which the material can be poured into or caused to flow into the mixer.
This type of batchwise volumetric dosing of expanded perlite is relatively inexact and can give rise to the escape of dust which can be removed only by the expensive application of suction.